Evaluation of Bond Strength of Pressed and Layered Veneering Ceramics to Nickel-Chromium Alloy.

STATEMENT OF THE PROBLEM
The success of metal- ceramic- restorations (MCR) depends on the presence of strong bond between porcelain and metal substructure.


PURPOSE
The purpose of this study was to evaluate the effect of hot pressing technique on the bond strength of a metal-porcelain composite in comparison to layering technique.


MATERIALS AND METHOD
Thirty Nickel-Chromium specimens were produced by two methods; conventional porcelain layering on metal and hot pressing (n=15). Bond strengths of all specimens were assessed by the means of three-point bending test according to ISO 9693: 1999 (E) instructions. The data were analyzed using Students t-test (p< 0.001).


RESULTS
The mean ± SD bond strength of conventional and hot pressing technique was 48.29 ± 6.02 and 56.52 ± 4.97, respectively. Therefore, the conventional layering technique yielded significantly lower mean bond strength values than hot pressing technique (p< 0.001).


CONCLUSION
This study showed that it is possible to improve metal-porcelain bond strength significantly by applying an overpressure during porcelain firing.


Introduction
Metal ceramic restorations are used extensively in dental prosthetics. [1][2] In these restorations, aesthetic qualities of the ceramic materials can be used in combination with the strength and toughness of metal alloys to produce restorations that have both aesthetic and proper mechanical features. [3][4][5][6][7] Various types of alloys have been introduced for the metal ceramic restorations. Nobel alloys containing primarily gold palladium and a small percentage of indium have proven to be the most reliable ones. However, the major shortcoming of these alloys is their high cost and their lack of adaptability with various systems of ceramic. The alloys that are used as the base metal, enjoy certain desirable features such as low cost, increased strength, toughness, and greater resistance to distortion.However, they sometimes reveal additional oxide formations, prove difficult to finish and polish since they have a low ductility and exhibit a greater casting shrinkage. [8] The presence of a strong bond between porcelain and metal substructure determines the success of the metal-ceramic restoration (M-CR). [9] It is also believed that the adhesion mechanism which exists between metal and porcelain is a micromechanical bond, which is compatible with the coefficient of thermal expansion (CTE), match vanderwals force and a proper metal oxi- dation and ion interdiffusion between metal and porcelain. [10][11][12][13] Extensive studies have been carried out on the metal-porcelain bond. [14] Some effects were studied including firing cycle and temperature, [15][16] thermal and/or mechanical cycling, [17][18] cooling rate, [19] opaque layer thickness, [20]

Materials and Method
In this experimental study, pressed ceramic to core materials was selected as the experimental groups, and layer ceramics to the core materials was defined as the control groups (n = 15 per each group).
The features, composition, and the manufacturer of the materials which were used in this study are summarized in Table 1   Hanau, Germany). After casting process, all molds were bench cooled. Carbide discs were used at low speed to remove sprues and separate metallic strips. Then, the metal strips were divested and cleaned by using airborne-particle abrasion. The surface of the specimens which would receive the ceramic was air borne-particle abraded once more by using 150-µm aluminum oxide particles at an angle of 45 • for 10 seconds from nearly 2 centimeter distant and under pressure of 2 bars. The metal strips were cleaned by using a steam cleaner device, and were dried at the temperature of the room.
Finally, the oxidation process was performed according to the instructions of the manufacturer. By using a graphite pencil, an area of 8×3 mm was marked on the metal strips and then the veneering ceramics were fired on the metal frames ( Figure 1).    For complete fixed dental prostheses, metalceramic restorations have been a preferred restoration because of their long-term clinical use. [1][2]35] In order to establish an optimum esthetic outcome, veneering ceramics are classically layered on metal or zirconia core materials. [36][37] One possible technique is to press veneering ceramics to the core materials. Although the pressing technique itself is not considered as a recent technology, a process for pressing ceramics to metal and zirconia cores using the lost-wax technique and glass-ceramic ingots has been recently developed.

Results
[38] The layering technique has been the principle  [38] although they evaluated the bond strength of pressed and layered ceramics to noble alloys. In contrast to our study, Venkatachalam et al. [26] and Schweitzer et al. [27] found no significant differences between the two techniques. This disagreement could be attributed to the difference between the metal and also the porcelain materials that have been used in their study and our research. In our study, the veneering porcelains were selected from the same manufacturer in order to reduce the study variables as much as possible. Therefore, not only CTEs of both porcelains were compatible with metal substructure(less than 1×10 ) but also opaque material and also its application method was the same for both groups. Moreover, they could not ascertain the effect that additional steps of divestment and sprue removal had on the debond strength values of pressed ceramic samples characterized by larger standard deviations (SD). Another significant aspect to consider in their studies was the CTE mismatch between pressed ceramic and metal, which was greater than 3×10 •C. It is recognized that CTEs differences of 1.7×10 •C or greater between metal and porcelain could cause shear stresses at the interface which could result in a weak metal-ceramic bond or to its ultimate failure. To assure that porcelain is under compression at the interface, the metal's CTE should be slightly higher than of the porcelain, but, perfectly their mismatch should not be greater than 1×10 •C. [5,7,13] However in our study the tested metal and porcelain materials were compatible in regard to CTE. [39] In the future, studies evaluating the bond strength with different combinations of metals and veneering materials, and also the effect of mechanical and thermal cycling protocols are suggested. Furthermore, various types of methodologies to assess the bond strength might be deliberated.

Conclusion
Within the limitation of this in vitro study, it may be concluded that the tested metal-ceramic composites reveal sufficient bond strength for optimum clinical performance of the restorations. The bond strength of metals to porcelains could be improved by hot pressing technique.